Automated deep-research providers were attempted on 2026-07-18 but were unavailable (Falcon/Edison HTTP 402; Perplexity-lite HTTP 401). This manual synthesis is based on the reviewed UniProtKB record, cached primary publications, and the current Reactome record.
ITIH3 encodes HC3, a secreted heavy-chain component of human pre-alpha-inhibitor. Its precursor is processed to a mature chain whose C-terminal Asp651 is ester-linked through chondroitin 4-sulfate to bikunin. Human biochemical studies establish that pre-alpha-inhibitor contains one 90-kDa heavy chain and one 30-kDa inhibitory chain [PMID:2476436, "each protein contained a single, identical, trypsin-inhibitory chain of 30,000 Da"]. A later purification study identifies that inhibitory component explicitly: pre-alpha-inhibitor "comprises bikunin (BK) responsible for antiprotease activity, covalently linked to a heavy chain H3" PMID:9188816. Thus HC3 is not itself an endopeptidase inhibitor.
HC3's defining function is participation in the hyaluronan-rich extracellular matrix. A direct study of purified human HC3 identified an N-terminal BX7B HA-binding sequence. The binding activity was cryptic in the intact mature chain but became strong after thermolysin generated an N-terminal fragment [PMID:11168393, "unmasking this HA-binding activity required most of H3P to be trimmed off at its C-terminal end"]. More decisively, reconstitution with purified human plasma components showed that HC2-containing proteins promote formation of covalent HC3-TSG-6 and then HC3-hyaluronan complexes [PMID:18448433, "both HC2 and TSG-6 are required for the transesterification reactions to occur"]. HC3 is therefore a transferable HA-associated heavy chain, while TSG-6 and HC2 supply the transfer mechanism.
Mouse studies independently found HC3 in physiological and pathological SHAP-hyaluronan complexes and in complexes formed in vitro [PMID:18293178, "All three HC isoforms are found in the SHAP-hyaluronan complexes of physiological or pathological origins as well as that formed in vitro"]. This ortholog evidence supports extracellular-matrix localization for human HC3, while the direct purified-human experiment establishes the biochemical capacity to form HC3-HA. No human disease or reproductive process was assigned because the available studies do not isolate an ITIH3-specific causal role in those processes.
The core function of ITIH3 is to provide the noninhibitory HC3 component of plasma pre-alpha-inhibitor and, following HC2- and TSG-6-dependent transesterification, an HA-associated heavy chain incorporated into HA-rich extracellular matrices. Hyaluronic acid binding is supported for a proteolytically exposed HC3 fragment and by HC3-HA complex formation; hyaluronan metabolic process, extracellular matrix, and extracellular space are the most defensible accompanying GO concepts. Protease inhibition belongs to bikunin, not ITIH3.
The direct HC3 binding, HC3 transfer, pre-alpha-inhibitor purification, chain-crosslink, and mouse SHAP-HA papers are available only as abstracts in the local cache. Their abstracts explicitly support the conclusions above but do not establish tissue-specific kinetics or disease causality. The platelet dense-granule annotation is target-specific in Reactome's current participant set, but its underlying PMID:1435334 record contains no accessible abstract or ITIH3-specific text. Exosome recovery is retained as non-core because the source is a proteomic preparation of a secreted-body-fluid fraction.